This invention relates to the field of packaging materials for foodstuffs, specifically to the field of packaging foodstuffs for microwave irradiation. In the past, such packaging contained the foodstuff and may have included a susceptor for concentrating thermal energy for heating or cooking the food contained in the package. Such packages typically did not protect the foodstuff from overheating or overcooking, other than in certain embodiments, to reduce or eliminate the concentration caused by the susceptor or in the folds of such packaging. One typical example is microwave popping of popcorn, which is conventionally done in a paper bag carrying a susceptor. Once the popcorn is popped it has been found that it is easily scorched by continued exposure to microwave irradiation. The prior art has heretofore not addressed such continued exposure of the foodstuff to overlong microwave irradiation.
The present invention overcomes this deficiency of the prior art by providing a structure which is initially substantially transparent to microwave irradiation (allowing normal microwave heating and cooking). Upon reaching a predetermined temperature, the structure of the present invention morphs, or changes its own form, to a microwave shielding structure, preventing further heating or cooking (or scorching) of the foodstuff.